1. Technical Field
The present invention relates to a printing medium feeding device feeding a printing medium downstream, a printing apparatus having the same to perform a printing operation on the printing medium, and a liquid ejecting apparatus.
The liquid ejecting apparatus is not limited to a printing apparatus such as a printer, a copier, or a facsimile that ejects ink from an ink jet printing head to perform the printing operation on a printing medium, but also includes an apparatus capable of ejecting a liquid corresponding to the ink from a liquid ejecting head corresponding to the ink jet printing head on the printing medium to deposit the liquid on the printing medium.
In addition to the printing head, examples of the liquid ejecting head include a color material ejecting head used to manufacture a color filter such as that used in a liquid crystal display, an electrode material (conductive paste) ejecting head used to form an electrode such as that used in an organic EL display or a field emission display (FED), a bio-organism ejecting head used to manufacture a bio chip, and a sample ejecting head as a precision pipette.
2. Related Art
A printing apparatus such as a facsimile or a printer includes a paper feeding device that feeds printing paper downstream, which is a printing medium. In addition, the paper feeding device includes a driving roller driven by rotation of a motor and a driven roller coming in elastic contact with the driving roller and following the rotation of the driving roller. When the ends of a rotation shaft of the driving roller are supported by bearings, the center portion of the rotation shaft of the driving roller may be bent downward due to load applied from the driven roller or the driving roller may be bent upward due to a reaction force (tensile force). To avoid these problems, there are known configurations in which the rotation shaft is supported between two shaft supporting portions (JP-A-2006-82439, JP-A-2001-341911, and JP-A-2000-281255).
If the rotation shaft has deviation in component precision, the deviation may cause displacement in the driving roller upstream or downstream every certain period at the rotating time, thereby deteriorating printing paper transport precision. FIGS. 6A and 6B show these problems. Reference numeral 41 denotes a discharge driving roller discharging the printing paper, reference numeral 42 denotes a discharge driven roller coming in elastic contact with the discharge driving roller 41 and following the rotation of the discharge driving roller, and reference numeral 40 denotes a rotation shaft of the discharge driving roller driven by rotation of a motor (not shown). A plurality of the discharge driving rollers 41 are arranged in a line direction of the rotation shaft 40 at an appropriate interval (for example, see FIG. 1 or the like). FIG. 6A is a diagram illustrating the rotation shaft 40 when viewed in a Y direction. FIG. 6B is a sectional view illustrating the rotation shaft taken along the line A-A shown in FIG. 6A. X, Y and Z directions denote a paper width direction, a paper feeding direction, and a direction perpendicular to the printing surface, respectively.
In FIG. 6A, H denotes a supporting position of the rotation shaft 40 and CL denotes a straight line passing through a shaft core (which is denoted by Ca) at two supporting positions H of the rotation shaft 40. As shown in FIGS. 6A and 6B, the deviation of the rotation shaft 40 is smaller at the positions close to the supporting positions H and is larger at positions near the point equidistant between the two supporting positions.
As shown in FIG. 6B, core positions of the rotation shaft 40 at the supporting positions H (denoted by 40A) are each denoted by Ca and a core position of the rotation shaft 40 at the center portion of the rotation shaft 40 (denoted by 40B) is denoted by Cb. In this case, the core position Cb is displaced along the circumference of a circle with a radius d centered about the core position Ca at the rotating time when viewed in the core direction of the rotation shaft 40. Reference numeral 40′ in FIG. 6B denotes a displaced rotation shaft 40B. In addition, if the rotation shaft 40 deviates, wherein the component of the Y direction at the displacement time is the component of the paper feeding direction, the precision of the paper feeding deteriorates.
For example, a transport device disclosed in JP-A-2006-82439 is configured so that as a rotation shaft, a transport driving roller is supported from the downstream side by an intermediate receiver in order to avoid the above-mentioned problems. However, the transport driving roller is pushed by the intermediate receiver as a result of the intermediate receiver receiving the pressing force of a transport driven roller so as to maintain the core position. Accordingly, a force by which a driven roller comes in contact with the driving roller has to be reduced. For example, as shown in FIGS. 6A and 6B, the discharge driven roller 42 is a roller in which cogs are attached to the circumference, and thus the force by which the discharge driven roller 42 comes in contact with the discharge driving roller 41 has to be reduced. With such a configuration, it is easy for the rotation shaft of the driving roller to move upward, thereby causing displacement in the component of the paper feeding direction, as described above.
Additionally, in JP-A-2001-341911 and particularly in FIG. 4, a supporting member (“intermediate supporting member 45”) restricting the entire circumference of the rotation shaft (“shaft body 40”) is shown. However, even with such a configuration, a displacement in the component of the paper feeding direction as in the foregoing description may occur due to a clearance between the rotation shaft and the supporting member. Moreover, restriction on the entire circumference of the rotation may induce shaft rotation load to increase in some cases.
In the configuration in which the rotation shaft is supported in the intermediate portion as in the known technique, it is difficult for displacement in the component of the paper feeding direction to occur while the rotation shaft continues to rotate in a certain direction (for example, forward rotation direction). However, since the rotation shaft of the roller repeatedly performs normal rotating motion and stopping of the motion in a printing operation, a displacement in the component of the paper feeding direction may occur at the time the printing operation stops. In particular, in a configuration in which paper is fed by a pair of rollers separated from each other in the paper feeding direction, a paper feeding speed can be configured to be higher in order to prevent the paper between the pair of rollers from becoming loose. At this time, since the roller on the downstream side receives a tensile force, reverse rotation may occur in the roller on the downstream side at the time the paper feeding stops. As a result, the displacement in the component of the paper feeding direction may occur as in the foregoing description.
The known printing apparatus is not configured so that the displacement in the component of the paper feeding direction caused by the rotation of the rotation shaft of the driving roller can be reliably prevented.